The aorta is the main artery of the body, arising from the base of the left ventricle of the heart. It is a large vessel, about one inch in diameter, with thick, elastic walls to withstand the changes in pressure as the heart beats. It supplies oxygenated blood to all arteries except the pulmonary artery (which carries blood to the lungs to be oxygenated). The aorta curves behind the heart and down the chest into the abdomen, where it divides into the two iliac arteries.
A pathologic dilation or balloon like enlargement of the aorta below the renal arteries may occur as a consequence of aging, atherosclerosis, infection, inflammation, trauma, congenital anomalies, or other pathologic conditions. Localized dilation of the aorta below the renal arteries to a diameter 50 percent greater than the expected normal diameter is known as abdominal aortic aneurysm, or AAA. Generally, aneurysms grow gradually over time, increasing by an average of two to three millimeters annually. It can take 10 to 15 years for an aneurysm to reach a size at which surgery is deemed necessary. Surgical repair of abdominal aortic aneurysm is indicated when the risk of rupture is believed to exceed the risk associated with open surgery. The risk of rupture increases with the size of the aneurysm. Due to the high risk of rupture for abdominal aortic aneurysms greater than 5 cm and the mortality associated with rupture, surgical repair of such aneurysms is the standard of care. At 5 to 6 cm, the risk of rupture increases by 20 percent per year.
Abdominal aortic aneurysm is the 13th leading cause of death in the United States accounting for more than 16,000 deaths annually. More than 40,000 procedures to treat abdominal aortic aneurysms are performed in the United States annually with approximately 32,000 new patients diagnosed with unruptured aneurysms each year. Of those diagnosed patients, 24,000 were over the age of 65. More than two million people are estimated to have an undiagnosed abdominal aortic aneurysm. Over the past 40 years, the prevalence of abdominal aortic aneurysm has risen three-fold.
Studies indicate that for aneurysms larger than 4 cm, there is a 3 to 4 percent prevalence in men age 65 to 80 and as high as 12 percent in elderly males with high blood pressure.
Generally, abdominal aortic aneurysms are asymptomatic and are often discovered only as a result of investigation of other medical problems. The primary risk posed by an abdominal aortic aneurysm is its propensity to rupture. Emergent surgical repair of a ruptured aneurysm has a high risk (40 to 50 percent) of perioperative mortality. Elective aneurysm repair, by contrast, has an associated mortality risk of 1 to 5 percent. Up to 5096 of patients with untreated aneurysms die of rupture in a 5-year period.
Medical therapy for abdominal aortic aneurysms is thought to be ineffective in preventing rupture. For the past 40 years, the standard treatment for abdominal aortic aneurysm has remained relatively unchanged: open abdominal repair. Open repair is contraindicated in many patients, usually because of advanced age and associated medical problems.
For open abdominal repair, the surgeon makes a large midline incision (8 to 10 inches) exposing the entire abdominal cavity, including the aorta and other organs. The bowel is retracted from the operative field and the aorta is visually examined to determine the proper size and configuration of the synthetic graft that will be used to replace the diseased vessel. If the iliacs are involved or if there is not an adequate amount of healthy aorta distal to the graft, then a bifurcated graft is utilized. The aorta or the aorta and iliacs are cross-clamped proximal and distal to the aneurysm for a period of 30 to 90 minutes and the diseased section is replaced by a prosthetic graft by suture. The wall of the aorta is wrapped and sewn around the graft to protect it. The incision site is then closed with sutures and staples.
The open surgery procedure can take up to four hours to perform and has significant morbidity rates in 15 to 30 percent of patients. The average hospital stay associated with open abdominal repair is seven to twelve days, including time spent in the ICU. Patient recovery time can last three months. Mortality rates for the surgical repair of non-ruptured aneurysms from multicenter reports have a range of 3–5% and at approximately 7% in population based studies.
In recent years, a less invasive approach for repairing abdominal aortic aneurysms has been developed: endovascular repair. An endovascular graft procedure provides essentially the same treatment as open abdominal repair, but via a different access route. The surgeon begins by making small incisions above the femoral arteries in the groin area of each leg. Using catheters, the interventional physician then inserts the stent graft through the femoral artery and precisely places the graft at the appropriate anatomical location. The graft is secured into place by the self-expansion of the nitinol stent rings. This seals off the distended portion of the aorta from circulation.
The first commercially available endoluminal graft was implanted in the early 1990s. This early tube graft was applicable to only 5 to 10 percent of patients with abdominal aortic aneurysm due to anatomical considerations. Subsequent technological innovations have included the development of a bifurcated stent graft and improvements in delivery systems, which expand applicability to 40 percent of patients with abdominal aortic aneurysm.
The placement of endoluminal grafts generally has required a team approach, the team may include a cardiologist, surgeon, radiologist, and support staff. More extensive pre-case evaluation is required than with open surgical repair due to necessary considerations such as endovascular access, implantation sites and morphology of the aneurysm. Proper patient selection and customization of the endoluminal graft require careful pre-procedural imaging to address these anatomical requirements. In addition, the surgeon must be trained in the use of guidewires, catheters and imaging modalities.
Compared to open surgery, the endovascular procedure provides substantial patient benefits such as reduced trauma from the surgery, shorter hospital stay, shorter intensive care unit stay, less blood loss, and fewer postoperative complications such as pulmonary compromise.
One problem with presently available stent/stent graft systems lies in the delivery and deployment of the stent/stent graft. Typically a series of interfitting catheters and tubes are provided to push or otherwise move the stent graft from the catheter and allow it to be deployed. In practice, the use of such a “push-pull” type system is less than ideal. Physicians have indicated a better delivery and deployment approach would be desired.
Recently, Fiedler in U.S. Pat. Nos. 5,817,101 and 6,056,759 and Monroe in U.S. Pat. No. 6,113,608 have proposed hydraulically operated systems to be used in deploying a stent. Both are complex in construction and include numerous pieces, further they present a multi piece large diameter short stent/stent-graft containment sheath that is not well adapted for the needs of insertion of a stent graft placement system which requires the ability to release with smooth reliability without kinking and without binding.